The conventional switching principle for switching on and off high currents in switching devices generally involves a double-interrupt contact arrangement, which guides the switch arcs occurring therein via arc guide rails in a stack arrangement of baffles in the form of deionizing chambers. In these chambers, the arcs are cooled and divided into a plurality of sub-arcs, this being linked to a corresponding multiplication of the arc voltage. When the driving voltage is achieved, the arc is quenched and the circuit is thus interrupted. When high alternating currents are switched, the arc quenching is generally assisted by dynamic magnetic blow fields, which are formed by suitably shaping the current conductors within the switching device. By contrast, for quenching direct currents, magnetic blow fields are generally used, which are generally produced by an arrangement of permanent magnets.
Unlike in the conventional AC switching devices which have long been on the market, comparably large switching devices for disconnecting low-frequency currents, for example at 16⅔ Hz, and direct currents are subject to correspondingly higher loads as a result of the reduced or absent periodicity of the current zero crossing. The resulting longer arc duration results in a higher energy content of the switch arcs than in AC switching devices. This leads to an increased burnup of contact material, and also to a correspondingly high thermal load within the switching chamber. A thermal load of this type can reduce the insulation capacity within a switching chamber. As a result, the electrical service life of the switching device may be reduced.
One option for reducing the load on a switching device from switch arcs is provided by hybrid switches, which consist of a parallel connection of an electromechanically actuated mechanical contact arrangement and a power semiconductor switch for example on the basis of a high-power IGBT (insulated gate bipolar transistor), as disclosed for example in German laid-open publication DE 10315982 A1. This has a high resistance when switched on, in such a way that the load current flows exclusively via the closed mechanical contacts. During the switch-off process, the power semiconductor is controlled in such a way that it briefly has a low resistance, in such a way that the arc current flowing through the mechanical switch is briefly commuted to the parallel power semiconductor switch; subsequently, this is controlled to block current again, causing the current commuted to the semiconductor to be rapidly brought to zero therein in an arc-free manner. Using a hybrid arrangement of this type, the effective arc time and thus the load on the switch can be greatly reduced.
To achieve a high electrical service life and acceptable dimensioning of the power semiconductor switch for high currents, it is expedient to limit the current flow time during the switch-off process. In air-operated switching arrangements, especially for high currents, this has the drawback that during the switching process using a typical mechanical bridge switching arrangement temporal fluctuations occur at an order of magnitude such that fully arc-free switching with only a brief current load on the power semiconductor switch can only be implemented with difficulty in practice.
This drawback can be overcome by using a vacuum switching chamber. Unlike with air switching, where the air in the region of the switch arc is ionized in part during the switching process, in a vacuum switching chamber a metal vapor arc of evaporating contact material is formed in a vacuum switching chamber when the contacts are disconnected under load, and condenses out in the interior of the vacuum chamber within a few microseconds in the zero-current case, resulting in virtually instantaneous reconsolidation of the switching path in the absence of an ionizable gaseous atmosphere.
Vacuum switching chambers, as disclosed for example in German laid-open publication DE 19902498 A1, usually consist of a connection electrode rigidly connected to the switching chamber housing and comprising a fixed contact at the inner end thereof and an opposite electrode comprising a sliding contact which is movable over a flexible metal bellows in an axial direction with respect to the fixed electrode in a vacuum-tight manner. Double-contact switches comprising vacuum switching chambers are known for example from German laid-open publications DE 38 11 833 A1 and DE 101 57 140 A1 and from US patent specification U.S. Pat. No. 8,471,166 B1.